US9640687B2 - Method for producing the P-N junction of a thin-film photovoltaic cell and corresponding method for producing a photovoltaic cell - Google Patents

Method for producing the P-N junction of a thin-film photovoltaic cell and corresponding method for producing a photovoltaic cell Download PDF

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US9640687B2
US9640687B2 US14/892,462 US201414892462A US9640687B2 US 9640687 B2 US9640687 B2 US 9640687B2 US 201414892462 A US201414892462 A US 201414892462A US 9640687 B2 US9640687 B2 US 9640687B2
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Giovanni Altamura
Louis Grenet
Simon Perraud
Frédéric Roux
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Definitions

  • the present invention relates to the field of photovoltaic solar energy and more particularly to thin film photovoltaic cells.
  • a ⁇ thin layer>> will be a layer having a thickness of less than 5 ⁇ m, or even less than 3 ⁇ m.
  • photovoltaic cells give the possibility of directly converting the light from the sun into electricity by using the electronic properties of suitable materials.
  • the manufacturing of photovoltaic cells involves the formation of a p-n junction between a semi-conductor of type p, in which light is absorbed, and a semi-conductor of type n.
  • a thin film photovoltaic cell generally comprises a substrate for supporting the stack of layers, this stack comprising a metal layer forming the lower electrode, an absorbing layer, of type n or p, for generating charges during absorption of light with minimum transmission and reflection losses, a buffer layer, of type p or n (the buffer layer being of the opposite type to that of the absorbing layer) in order to create the junction with the absorbing layer with a minimum of absorption loss and a transparent conducting electrode for bringing the current to an external circuit.
  • Each material has particular physical and chemical properties which have an influence on the overall performance of the photovoltaic cell.
  • any defect at the interface between the buffer layer and the absorbing layer forms an area where the electric charges may recombine. Consequently, it is essential to be able to control the formation of the p-n junction within a photovoltaic cell, in order to avoid the formation of defects.
  • solar cells in thin layers have advantages as compared with solar cells made in crystalline silicon. This notably results from the fact that they require less material since their absorbing layer has a better optical absorption coefficient.
  • the absorbing layer of a photovoltaic cell in thin layers may notably be made in a material of the CIGS type, of general formula Cu(In,Ga)(S,Se) 2 , CdTe or further CZTS, of general formula Cu 2 ZnSn(S,Se) 4 .
  • this yield being defined as the percentage of solar energy to which the cell is exposed which is converted into electric energy.
  • the energy conversion yield is of the order of 15% for a material of the CdTe type and of the order of 10% for a material of the CZTS type.
  • the p-n junction is made by, first of all, forming the absorbing layer in a material of the CIGS, CZTS or CdTe type and in a second phase the buffer layer.
  • the surface of the absorbing layer may be contaminated with oxygen. This is particularly the case when the buffer layer is formed by a chemical bath deposition technique (CBD).
  • CBD chemical bath deposition technique
  • the period of time separating the formation of the absorbing layer and the deposition of the buffer layer plays a major role in the final performances of a photovoltaic cell.
  • a solar cell may have a structure of the substrate or superstrate type.
  • the manufacturing of the solar cell begins by forming on a substrate a metal layer forming the lower electrode on which is then made an absorbing layer, for example of type p.
  • This absorbing layer may notably appear as a thin film in CIGS or CZTS.
  • the buffer layer in a semi-conducting material of type n, is then deposited on the absorbing layer.
  • the cell is finished by depositing a conductive transparent electrode.
  • the same stack of layers may also be obtained by depositing the layers in the opposite direction, so as to obtain a structure of the superstrate type.
  • the manufacturing of the solar cell of the superstrate type begins by depositing a conductive transparent electrode on a transparent substrate.
  • a buffer layer of type n is then deposited on this conductive transparent electrode, an absorbing layer of type p then being formed on the buffer layer.
  • the manufacturing of the solar cell is ended by forming a metal layer forming a rear electrode.
  • the major advantage of the structure of the superstrate type is that no oxidation of the absorbing layer may occur at the interface with the buffer layer.
  • the formation of the absorbing layer is carried out at relatively high temperatures. This may lead to chemical diffusion from the buffer layer into the absorbing layer.
  • the energy conversion yield of a solar cell having a structure of the superstrate type comprising a p-n junction between an absorbing CIGS layer and a buffer CdS layer is limited to about 7%.
  • Solar cells having a structure of the superstrate type with a buffer layer in ZnS or ZnO have better yields, for example of the order of 11% in the case of ZnO.
  • these materials nevertheless lead to significant chemical diffusion from the buffer layer into the absorbing layer.
  • the p-n junction is always formed in two steps, the absorbing layer and the buffer layer being made independently of each other.
  • This method has the advantage of being able to be easily applied on an industrial scale since it may be integrated into a completely online system leading to significant production volumes.
  • this method gives the possibility of avoiding oxidation of the surface of the absorbing layer.
  • the p-n junction is always formed in two successive steps.
  • PVD physical vapor deposition
  • cadmium is a toxic element.
  • the object of the invention is to overcome the drawbacks of known methods by proposing a method for making a p-n junction in a thin film photovoltaic cell which gives the possibility of avoiding the formation of defects at this junction notably resulting from oxidation, which is of a simplified application and which may give the possibility of avoiding the use of cadmium.
  • the method according to the invention comprises a deposition step in which are successively made: a layer of precursors of a photovoltaic material of type p or n, a barrier layer and a layer of precursors of a semi-conducting material of type n or p, this deposition step being followed by a single annealing step carried out with a supply of S and/or Se, this annealing step leading to the formation of an absorbing layer of type p or n and of a buffer layer of type n or p and of a p-n junction at the interface between said layers, the barrier layer preventing chemical diffusion and oxidation at the interface between the absorbing layer and the buffer layer during said annealing step.
  • This method gives the possibility of both avoiding oxidation and chemical diffusion at the interface between the absorbing layer and the buffer layer while producing the photovoltaic layer and the buffer layer during the same annealing step. This therefore leads to a p-n junction of very good quality with a method for which the application is simplified.
  • the precursors of a photovoltaic material of type p or n comprise metal precursors of Cu, Zn, Sn or of Cu, Ga and In.
  • a layer of Se and/or S is deposited on the layer of precursors of a photovoltaic material of type p or n, before deposition of the barrier layer.
  • the precursors of a photovoltaic material also comprise selenium and/or sulfur compounds of said metal precursors.
  • the precursor of a semi-conducting material of type n or p is indium and leads to the formation of a buffer layer in In 2 S 3 .
  • the sulfur and selenium are advantageously provided in gaseous form.
  • the annealing step is carried out, preferably, at a temperature comprised between 400 and 650° C., with temperature raising ramps comprised between 1° C./s and 15° C./s.
  • the barrier layer is formed with a nitride or oxide. It is advantageously a layer of Si 3 N 4 , SiO 2 or TiO 2 .
  • the invention also relates to a method for producing a thin film photovoltaic cell.
  • This method comprises the following steps:
  • the rear-face electrode is a molybdenum layer.
  • an intrinsic ZnO layer is deposited before applying the method for producing a p-n junction according to the invention and before producing the conductive transparent electrode.
  • This transparent and conductive electrode is advantageously made in ZnO doped with Al.
  • the invention also relates to a thin film photovoltaic cell successively comprising a rear-face electrode, an absorbing layer in a photovoltaic material of type p or n, a barrier layer, a buffer layer of the n or p type and a front-face transparent and conductive electrode, with a p-n junction at the interface between the absorbing layer and the buffer layer.
  • the barrier layer is formed with a nitride or oxide and the buffer layer is made in In 2 (S, Se) 3 or further in Zn(O, S) or Zn(O, Se).
  • FIG. 1 is a sectional view of a stack of layers obtained after the deposition step of the method according to the invention
  • FIG. 2 is a sectional view of a stack of layers obtained after the annealing step of the method according to the invention
  • FIG. 3 is a graph giving the current density versus the voltage for a solar cell obtained by the method according to the invention.
  • FIG. 4 is a graph giving the current density versus the voltage for a solar cell similar to the one which is the subject of the graph of FIG. 3 but obtained by a conventional method.
  • the method for producing a photovoltaic cell according to the invention first of all consists of depositing on a substrate 1 , a metal layer 11 forming a rear-face electrode for the photovoltaic cell which will be obtained by the method according to the invention.
  • the substrate 1 may be made in various materials, conventionally in glass, in plastic or in metal. Generally, this substrate is made in a sodalime glass and has a thickness comprised between 1 and a few millimeters of thickness. Flexible, metal or plastic substrates may also be contemplated.
  • the layer 11 is for example made in molybdenum and its thickness is comprised between 100 nm and 2 ⁇ m and it is notably equal to 500 nm.
  • the deposition of the molybdenum layer may notably be achieved by cathode sputtering.
  • the precursors which will lead to the formation of the photovoltaic layer are then deposited, as a layer 12 , the precursors which will lead to the formation of the photovoltaic layer.
  • metal precursors of Cu, Zn, Sn and optionally at least one element taken from Se and S are metal precursors of Cu, Zn, Sn and optionally at least one element taken from Se and S.
  • the ratios of the Cu, Zn and Sn elements are conventionally selected so that 0.75 ⁇ Cu/(Zn+Sn) ⁇ 0.95 and 1.05 ⁇ Zn/Sn ⁇ 1.35.
  • This layer 12 may essentially include metal precursors.
  • the sulfur and/or selenium are then provided in gaseous form.
  • They may also be provided in the form of a layer 13 deposited on the layer of metal precursors.
  • the layer 12 may be a layer of metal precursors and/or of their corresponding selenium and/or sulfur compounds.
  • the layer 12 may be a layer of Cu, Zn and Sn and/or of their selenium or sulfur compounds such as CuSe, CuS, ZnSe, ZnS, SnSe, SnS, or any combination of these compounds. In this case, it is not necessary to provide a layer 13 consisting of sulfur and/or selenium.
  • provision of selenium and/or sulfur is preferred in the form of a deposit in the stack. Indeed, it is then easier to control the final composition of the photovoltaic layer than with a provision in gaseous form.
  • the layer 12 may include three successive layers, a ZnS layer, a copper layer and a tin layer, the order of these layers may be modified.
  • the thickness of the ZnS layer may then be of about 350 nm, that of the copper layer of about 120 nm and that of the tin layer of about 160 nm.
  • the layer 13 may be a selenium layer and have a thickness of about 2 ⁇ m.
  • These may be vacuum processes, such as evaporation or cathode sputtering or processes applied at atmospheric pressure, such as electrodeposition, screen printing, doctor-blading, inkjet or slit-coating.
  • a barrier layer 14 is then directly deposited on the layer of precursors 12 or on the layer 13 of S and/or Se when it is provided. This layer is continuous.
  • this barrier layer gives the possibility of avoiding chemical diffusion of components and oxidation.
  • This barrier layer is formed with a compound based on a nitride or oxide. It may notably be made in Si 3 N 4 , SiO 2 or TiO 2 . Its thickness is from a few nanometers and for example equal to 4 nm.
  • this layer 15 may notably be made in indium or zinc and its thickness will be from a few tens of nanometers and for example of about 30 nm.
  • this stack is made, it is subject to an annealing step, carried out with a provision of S and/or Se.
  • the chalcogen S or Se may then be provided as an elementary gas or as a gas of the H 2 S or H 2 Se type.
  • This annealing step leads to the formation of an absorbing layer 22 in photovoltaic material and of a buffer layer 25 .
  • This annealing step does not modify the barrier layer 14 .
  • This annealing step is carried out at a high temperature.
  • This annealing step may notably be carried out by using a temperature raising ramp comprised between 1° C./s and 15° C./s so as to obtain a temperature comprised between 500 and 650° C. and for example equal to 550° C.
  • the duration of the plateau at a high temperature may be comprised between 30 seconds and 30 minutes, and it is advantageously equal to 3 minutes.
  • This temperature allows crystallization of the metal precursors in CIGS or CZTS, in order to obtain the layer 22 .
  • This annealing step also allows transformation of the precursor(s) present in the layer 15 into a material forming the buffer layer 25 .
  • the layer 25 is then formed in In 2 (S, Se) 3 or further in Zn(O, S) or Zn(O, Se).
  • the annealing step leads to a layer 22 in CZT(S, Se) with a thickness of 1.2 ⁇ m, while the layer 25 in In 2 (S, Se) 3 is of about 50 nm.
  • the barrier layer 14 prevents the diffusion of the components present in the layer 12 , notably of indium or zinc, into the photovoltaic material of the layer 22 . It therefore gives the possibility of avoiding oxidation between the absorbing layer and the buffer layer during annealing.
  • this annealing step may be accompanied by the formation of a layer of Mo(S, Se) 2 at the interface between the molybdenum layer 11 and the layer 22 in a photovoltaic material.
  • the thickness of the Mo(S, Se) 2 layer depends on many parameters, notably on the amount of provided S/Se. It may be estimated to be comprised between 5 and 100 nm. It is known that its presence promotes contact of the ohmic type at the interface with the absorbing layer 22 , notably in CIGS.
  • the formation of a photovoltaic cell further requires the deposition of a transparent and conductive front-face electrode (not shown in the figures).
  • This front-face electrode is made in a transparent and conductive material.
  • It may notably consist of ZnO doped with Al.
  • the thickness of this layer is typically comprised between 100 and 800 nm and preferably equal to about 500 nm.
  • This transparent and conductive electrode may notably be deposited by cathode sputtering.
  • this layer forming the front-face electrode, it is possible to deposit on the buffer layer 25 , a layer of a material transparent in the solar spectrum and strongly resistive.
  • This layer is typically made in intrinsic ZnO and may have a thickness of a few tens of nanometers, and notably 50 nm for the solar cell given as an example earlier.
  • the constitutive material of this layer may typically be deposited by cathode sputtering.
  • This layer is optional and it may be omitted without causing any significant degradation of the electrical performances of the photovoltaic cell.
  • FIGS. 3 and 4 are graphs giving the current density (in mA/cm 2 ) versus voltage (in V).
  • this is a solar cell for which the lower or rear-face electrode is a molybdenum layer, for which the thickness is 500 nm, an absorbing layer in CZT (S, Se) for which the thickness is of about 1.2 ⁇ m, a barrier layer in Si 3 N 4 for which the thickness is of about 4 nm, a buffer layer in In 2 (S, Se) 3 with a thickness of about 50 nm, a ZnO layer for which the thickness is of about 50 nm and finally a ZnO layer doped with Al for which the thickness is of about 500 nm.
  • the lower or rear-face electrode is a molybdenum layer, for which the thickness is 500 nm
  • an absorbing layer in CZT (S, Se) for which the thickness is of about 1.2 ⁇ m
  • a barrier layer in Si 3 N 4 for which the thickness is of about 4 nm
  • a buffer layer in In 2 (S, Se) 3 with a thickness of about 50 nm
  • This solar cell is obtained with the method according to the invention.
  • the solar cell includes the same layers, except as regards the barrier cell which is absent.
  • FIGS. 3 and 4 shows that each solar cell according to the invention operates in the same way as a similar solar cell, obtained by a conventional method and not including any barrier layer.
  • FIG. 3 shows that it is possible to obtain separation of the charges even if a barrier layer is provided at the p-n junction. In practice, this separation of the charges is possible since the electrons present in the conduction band are able to cross the barrier layer by the tunnel effect.
  • the two following quantities remain unchanged: the open circuit voltage Voc (in V) and the short-circuit current density Jsc (in mA/cm 2 ).
  • Voc a value of 0.5263 V is measured for the cell of FIG. 3 and a value of 0.5228 V for the cell of FIG. 4 .
  • Jsc a value of 45.2830 mA/cm 2 is measured for the cell of FIG. 3 and a value of 45.3024 mA/cm 2 for the cell of FIG. 4 .
  • the corresponding values for the cell of FIG. 3 obtained by the method according to the invention is 63.27% and 15.08%. These values are respectively 78.46% and 18.58% for the cell of FIG. 4 .
  • the energy conversion yield of the solar cell of FIG. 3 is greater than the one obtained for solar cells having a structure of the superstrate type or for which the p-n junction is obtained by a vacuum process, all these cells avoiding oxidation of the absorbing layer.
  • the method according to the invention gives the possibility of producing in a single annealing step both the absorbing layer and the buffer layer of a solar cell.

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US14/892,462 2013-05-24 2014-05-23 Method for producing the P-N junction of a thin-film photovoltaic cell and corresponding method for producing a photovoltaic cell Expired - Fee Related US9640687B2 (en)

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FR1354696A FR3006109B1 (fr) 2013-05-24 2013-05-24 Procede de realisation de la jonction p-n d'une cellule photovoltaique en couches minces et procede d'obtention correspondant d'une cellule photovoltaique.
FR1354696 2013-05-24
PCT/IB2014/061659 WO2014188386A1 (fr) 2013-05-24 2014-05-23 PROCEDE DE REALISATION DE LA JONCTION p-n D'UNE CELLULE PHOTOVOLTAIQUE EN COUCHES MINCES ET PROCEDE D'OBTENTION CORRESPONDANT D'UNE CELLULE PHOTOVOLTAÏQUE.

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FR3006109A1 (fr) 2014-11-28
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FR3006109B1 (fr) 2016-09-16
WO2014188386A1 (fr) 2014-11-27
US20160104808A1 (en) 2016-04-14

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